A canned message is a message that you have already defined so that you do not have to type it while communicating with your website visitor. As typing a chat message consumes time and while you are talking business, every second counts. So you can pre-define a wide range of most frequently used chat messages as canned messages. This helps your operators entertain more visitors in less time.
You need to be in a Live Chat session with a visitor to be able to send the canned messages.